In recent years, power systems have been increased in voltage and current and the capacities of gas circuit breakers have been increased to obtain required interrupting performance. Meanwhile, downsizing by the optimization of interrupting portion structures and exhaust/shield structures has also been pursued for the purpose of cost reduction.
FIG. 2 illustrates the general structure of a gas circuit breaker. The gas circuit breaker is housed in a tank 1 filled with insulating gas. Under normal conditions, a fixed arching contact 3 on the electrode side and a moving arcing contact 5 on the moving side are electrically connected with each other. When an opening operation is instructed at the time of an accident, the moving side is actuated by an actuator through an insulating rod 10. As a result, the fixed arching contact 3 on the electrode side and the moving arcing contact 5 on the moving side are caused to transition into a physically open state.
Even after the contacts are opened, a current flows and an arc is produced between the fixed arching contact 3 and the moving arcing contact 5. The gas circuit breaker blows high-pressure insulating gas on the arc to extinguish the arc. For the purpose, when the moving side is actuated, the insulating gas in a puffer chamber 9 is compressed by a fixed piston 6. Then the gas is blown on the arc and the arc is extinguished.
The hot gas produced during gas blowing is high in temperature and low in density and is thus low in dielectric strength. For the prevention of degradation in dielectric strength between electrodes, after success is achieved in arc-extinguishing, it is necessary to swiftly discharge the hot gas from between the electrodes through an exhaust tube 2.
The roles of the exhaust tube are to swiftly discharge produced hot gas without retaining the hot gas and to efficiently cool the hot gas.
A description will be given to the mechanism of the occurrence of electrical breakdown between the exhaust tube 2 and the tank 1 with reference to FIG. 2. When gas is insufficiently cooled and hot gas high in temperature and low in dielectric strength arrives at the high electrical field portion at an end of the exhaust tube with the density of the gas remaining low, the following takes place: the dielectric strength between the exhaust tube 2 and the tank 1 is degraded. As a result, a fault (earthing) occurs to cause electrical breakdown between the exhaust tube 2 and the tank 1.
To cope with earthing faults, various means are taken. For example, the gas tank diameter is extended to obtain high dielectric strength due to electric field relaxation between the exhaust tube and the tank, or the exhaust tube is expanded to enhance hot gas cooling performance.
In addition, a through hole is provided in the exhaust tube to draw high-density, low-temperature gas into the exhaust tube through the through hole utilizing the pressure difference between inside and outside the exhaust tube, or a spiral groove structure is provided in the inner circumferential surface of the exhaust tube to prevent low-density insulating gas from being brought into contact with the inner circumferential surface in proximity to an end of the exhaust tube. Degradation in dielectric strength is thereby prevented (Patent Literature 1).